When drilling a borehole through a geologic formation, it is important to know the downhole conditions to ensure that the drill bit is operating correctly. These conditions include, among others, the diameter of the borehole and, therefore, the volume of the drilling fluid at any given point. In addition, the formation properties are measured to predict the presence of oil or gas. Formation properties may be logged with wireline tools, logging while drilling (LWD) tools, or measurement while drilling (MWD) tools. Modern oil and gas explorations typically use LWD or MWD tools, instead of wireline tools, for formation logging due to the saving in time and costs.
Various LWD and MWD tools are in use for measuring borehole or formation properties. For example, LWD neutron or gamma spectroscopy logs are used to provide lithology, formation porosity, and formation density information. Neutron/gamma spectroscopy is often performed by sending a pulse of neutrons into the formation using a pulsed neutron generator (PNG). The neutrons interact with elements in the formation by inelastic interactions or elastic interactions. The high-energy neutrons gradually lose their energy through these interactions to become thermal neutrons, which may be captured by the nuclei of various elements in the formation. After neutron capture, these elements become activated. The activated elements then decay by emitting gamma rays. The gamma rays emitted by these activated elements may be detected with gamma ray detectors. Because different elements produce gamma rays of different energies, the captured gamma ray spectra may be used to derive the elemental compositions of the formation. The elemental yields in turn may be used to provide formation lithology because different sediment layers are typically enriched with different types of elements. Methods for neutron and gamma ray logging are well known in the art. Detailed descriptions may be found in, for example, U.S. Pat. No. 5,440,118 issued to Roscoe, U.S. Pat. No. 5,786,595 issued to Herron et al., and U.S. Pat. No. 5,539,225 issued to Loomis et al. See also Albertin et al., “The many facets of pulsed neutron cased-hole logging,” Schlumberger Oilfield Review, v. 8, no. 2, p. 2841, 1996.
However, various LWD or MWD tools used in formation logging are adversely affected by the presence of drilling fluids (muds) and their sensitivities are typically compromised by tool “stand offs,” i.e., the distances from tools (or sensors) to the borehole wall. For example, chloride ions in the drilling muds may interact with (capture) thermal neutrons with high efficiency reducing the sensitivity of the gamma spectroscopy. Therefore, LWD measurements often need to be corrected for the adverse effects from the drilling fluids or tool stand offs. To correct the effects of the drilling muds or tool stand offs, it is necessary to determine the borehole diameters, tool stand offs, or the mud hold up volumes at the sites of measurements while the borehole is being drilled.
Borehole diameters are typically measured using caliper tools. Various caliper tools are available in the art. However, most of these tools are useful only as wireline tools; they cannot be deployed while drilling. With wireline tools, these measurements are acquired after the drill strings have been pulled from the boreholes. There would be substantial time lags between the times when the boreholes are drilled and the formations are logged and when the borehole diameters are determined. During this period, the shapes and sizes of the boreholes might have changed due to borehole instabilities. For this reason, it is desirable that the borehole diameters are measured while the formations are logged during the drilling process. It is also desirable that the processes of determining the borehole diameter not interfere with the normal logging while drilling processes.
Furthermore, large quantities of drilling fluids are pumped through the drill strings into the boreholes while the boreholes are being drilled. The drilling fluids help cool the cutting surfaces of the drill bits and help carry out the earth cuttings from the bottom of the borehole when they flow up the annulus to the surface. To prevent formation fluids from flowing into the borehole during the drilling process, the drilling fluids are pumped under a pressure that is slightly higher than the expected formation pressure. The higher hydraulic pressure of the drilling fluids may result in a substantial loss of fluid into the formation when a permeable and low pressure zone of the earth formation is encountered. Detection of such fluid loss may be used in correction of the measurements of various LWD sensors. Fluid loss into the formation may be detected by the reduced flow back of the drilling fluids on the surface. However, for determining in what zone the fluid loss is occurring, means of detecting volumetric flows along the axial depth of the borehole are needed.
Time-of-flight measurement of activated slugs of fluid have been used in the prior art in connection with the Water Flow Log (WFL). In the WFL service, a slug of mud is activated and then timed over a relatively long duration. In this process, the PNG is normally off, and is activated only very briefly to periodically tag a slug of fluid with a neutron burst. Such a process does not match well with the LWD environment or with neutron tools, where the PNG remains activated most of the time.
Therefore, it would be desirable to have LWD-compatible methods and apparatus for determining fluid time-of-flight, borehole diameter, volumetric flow rate, and various other parameters at a given depth in the borehole.